The present invention relates to the bidimensional correlation in real time of an image obtained line by line and a stored image. The correlation device supplies signals for correlating the image for certain numbers of lines with the stored image in the time corresponding to a line scanning.
The correlation device according to the invention is more particularly applicable to systems carried by a vehicle and which supply images such that the lines are repeated through the advance of the vehicle. It is more particularly applicable to imaging by radar, sonar or optics which must necessarily function in real time and for which there is a high image line recurrence rate, as well as to systems for which the volume and consumption of the means used must be reduced to the greatest possible extent. Examples of such systems are vehicle-carried systems for guiding, marking with reference points and recalibrating maps.
For example, in the field of submarine acoustic imaging high definition sonar systems are used for visually displaying the sea bed. In the field of aerial cartography, airborne radar systems or active or passive infrared systems are used.
These systems comprise a transmitting antenna which transmits signals in the form of infrared, electromagnetic or ultrasonic waves into all or part of the surrounding space. The signals received by the same antenna are processed in order to separate the energies coming from the different directions. The separation distance obtained is dependent on the angular resolution of the antenna, which is a function of the ratio between the wavelength .lambda. of the transmitted signals and the length L of the antenna, i.e. .lambda./L.
For example, in order to obtain a high resolving power it is known to use a side-looking radar antenna functioning as a multiple antenna, i.e. using the displacement of the carrying vehicle for synthesizing a greater antenna length.
In airborne systems, for carrying out aerial cartography by radar using a multiple side-looking antenna, the signals received are recorded on photographic film and then processed to restore the true image. Processing consists of correlating the signals with the reference signal which is a function of the vehicle displacement and the distance from the object. Consequently, a large quantity of data are collected and correlation takes a long time. These operations are carried out optically by reading the film in the manner described e.g. in an article by L. J. Cutrona et al (Proceedings IEEE, Vol. 54, No. 8, 1966, p. 1026).
In other applications using radar signals, where the correlation functions and also convolution functions play an important part, processing takes place digitally, because the precision and flexibility levels are higher. These operations are mainly directed at measurements of the arrival, classification and identification times of the signals. Bearing in mind the calculation speed, the digital devices have significant overall dimensions and an excessive power consumption for airborne or submarine systems.